The invention disclosed herein relates generally to solenoid actuated systems, and more particularly to a servo system employing a proportional solenoid actuator and associated closed loop drive circuitry.
Solenoid actuators are characterized by a number of features which make them attractive for many applications requiring conversion of electrical energy to linear mechanical motion. Such actuators are inherently simple and rugged. Since they convert electrical energy directly to linear motion, a minimum number of moving parts are required. Further, relatively high forces may be obtained from a device of small physical size and power requirements, particularly at the energized end of its stroke.
Nevertheless, although proportional solenoids are known as shown in U.S. Pat. No. 3,154,729 issued to B. Duff on Oct. 27, 1964, solenoid actuators have not been commonly used in applications requiring proportional position control. One reason is that conventional solenoid actuators inherently do not produce a uniform force throughout the stroke distance. Accordingly, for anything approaching reasonably constant force operation, the drive current must be varied with armature position. Even then, with conventional drive circuitry it is difficult to achieve an acceptably uniform force throughout the length of the stroke.
Various schemes have been devised for modifying the force versus travel relationship in solenoid mechanisms. U.S. Pat. No. 1,754,069 issued to H. Trencham et al on Apr. 8, 1930 discloses a solenoid operated mechanism which utilizes two solenoids interconnected by a floating lever and working in unison to tailor the force and motion produced by the mechanism. The solenoids are operated in a predetermined sequence by a system of switches in which a switch for one solenoid may be controlled by the other solenoid. Although such a mechanism is capable of varying the normal operating characteristics of a solenoid actuated mechanism, the mechanization is somewhat cumbersome, and does not avoid the abrupt motions of conventional solenoid mechanisms.
Also, most common types of solenoids use magnetic attraction for armature movement in one direction only. A spring is typically employed to return the armature when the solenoid is not energized. Such designs, which include only a single electrical energization coil, are characterized by actuation forces which are different for different directions of armature travel.
Exceptions to this mode of operation are found in previously identified U.S. Pat. Nos. 3,154,729 and 3,766,432 issued to I. Markowitz et al on Oct. 16, 1963. However, no drive circuitry is described in the first of these patents, so that it cannot be ascertained how the disclosed solenoid should be energized. More particularly, the patent contains no suggestion of closed loop operation. The second of these patents discloses a bi-directional solenoid actuator with a pair of coils which produce oppositely directed forces on the solenoid armature to achieve rapid repositioning thereof, and holding of the armature in its desired position without the necessity of a high holding current for keeping the armature in place against a spring bias. Drive circuitry is disclosed which initially provides a relatively high switching current, and after the armature is seated, reduces the current to a relatively low holding value. The features of this drive circuitry are not, however, applicable to proportional positioning.
Other forms of drive circuitry for solenoid systems are shown in U.S. Pat. Nos. 3,289,191 issued to R. Schauer on Nov. 29, 1966 and 4,059,844 issued to J. Stewart on Nov. 22, 1977. These patents basically disclose solenoid drive circuitry schemes in which opposing polarity current drivers are utilized to energize a selected one of two or more solenoids, and to limit solenoid power requirements.
In part because of the foregoing characteristics of solenoid actuators, other types of actuators, such as stepper motors, rotary servo motors and motors with synchro shaft angle transmitters, have normally been used where proportional position control is needed. Typical systems of these types are disclosed in U.S. Pat. Nos. 2,549,829 issued to E. Lilja on April 24, 1951 and 3,510,739 issued to B. Peterson on May 5, 1970. Such actuators are inherently more complicated and expensive than solenoid actuators. Since there are many proportional position control applications in which solenoid actuators are potentially suitable, it would be desirable to provide a solenoid actuator and drive system for enhancing proportional solenoid operation and, for optimizing solenoid servo system performance.
The applicant has devised means for extending the applications in which a solenoid actuated mechanism is useful, while retaining the advantageous characteristics of such mechanisms, by employing close loop drive circuitry which provides servo operation. Armature position and force are independently controlled by input signals of a form which may be easily programmed to the characteristics of a particular solenoid actuator and to the operating parameters desired for a particular application.